Such apparatus conventionally comprises a hollow core extending longitudinally through the extrusion nozzle, into the mold tunnel and extending rearwardly through the extrusion head. Such a hollow core may act as an inner mandrel about which extrudate flows in an annular channel defined between the core and the outer casing of an extrusion nozzle. Extrudate may be introduced into the channel by a lateral opening thereinto and/or by an axial opening thereinto or when more than one channel is provided for plastic flow, by either or both lateral or axial openings.
The core performs various important functions. Within its hollow interior, it may carry pipes carrying blowing air to press the parison against the mold tunnel, pipes carrying cooling fluid to cool and help set thermoplastic parison located against the mold tunnel, pipes carrying heating fluid, if desired, pipes carrying suction for application to the inner surface of the parison, if desired, pipes carrying lubricant, hydraulic fluid, etc. The outside of the core may carry a forming plug located within the mold tunnel for forming the interior surface of the parison and maintaining its shape until the thermoplastic material is sufficiently set to retain it shape without assistance. Such a plug may be a cooling plug to aid setting of the thermoplastic material. It may be lubricated at its surface, may have applied suction at its surface, may provide other services at its surface from service pipes within the core. A heating plug and follower plugs may, in some circumstances also be carried by the core.
When the tube is to be corrugated pipe various aids for pressing the pipe into the corrugators of the mold tunnel may be carried by the core.
The core may carry at least one spider to carry services from the core to the interior of an outer parison and/or as a spacer for annular channels of the extrusion nozzle.
Still further the core may carry an inner die lip defining, with an outer lip an extrusion orifice opening for an angled exit passage the geometry of which is of considerable importance in different applications. Alternatively, the upstream end of the cooling plug may be shaped to form the appropriate angle for the exit passage.
At the downstream end of the core means may be provided to allow for slight longitudinal movement to adjust, for example, the width of the exit passage and means may also be provided to inhibit twisting of the core, there causing torsion of the parison.
The outer casing of the extrusion nozzle may be coaxial with the core and assembled with the extrusion head downstream thereof. The outer casing of the extrusion nozzle carries shaping die lips which often flare outwardly from the general diameter of the extrusion nozzle.
When double walled tubing is to be produced a hollow mandrel is present intermediate between the outer casing of the extrusion nozzle and the inner core and coaxial with them. This mandrel also carries die lips at its extrusion orifice.
All these coaxial integers may require replacement from time to time for pipe size changes or for other functions.
Of these functions, replacements in the form of tool changes for change of pipe size are perhaps the most common. For different sizes of tube it is frequently necessary to replace the die lips, the forming plug and, indeed the core itself. When double walled tube is involved change of the intermediate mandrel may be necessary. It is sometimes necessary to add a new part to carry out an additional function.
For such tool changes, it is, with conventional equipment necessary to disassemble the core and extrusion nozzle from the extrusion head and mold tunnel.
Since the core may carry parts on its outer surface, which parts may be of greater diameter than the core itself, and because the outer casing of the extrusion nozzle is attached to the extrusion head downstream thereof, has not been easy to withdraw the core and extrusion nozzle from the apparatus.
Indeed to access some components it has been necessary to disassemble the whole of the extrusion head involving a large amount of time and energy.